A variable valve timing device of an internal combustion engine, which is provided with an intermediate lock mechanism that enables a phase of a camshaft relative to a crankshaft to be locked at an intermediate phase, is well known in the art.
For instance, the variable valve timing device of the internal combustion engine, disclosed in Patent document 1, has a housing configured to rotate in synchronism with the crankshaft, a rotor installed in the housing and configured to rotate together with the camshaft, and two lock pins, each of which is configured to be engageable with both the housing and the rotor. Also disclosed is an intermediate lock mechanism configured to restrict relative rotation of the camshaft with respect to the crankshaft and hold valve timing at a predetermined intermediate lock position, by inserting the top ends of the two lock pins from the housing side into respective lock grooves formed in the rotor.
The variable valve timing device of the Patent document 1 is configured to be changeable the valve timing by supplying operating oil (working hydraulic fluid) to a phase-advance side hydraulic chamber or a phase-retard side hydraulic chamber, defined between the housing and the rotor. When the intermediate lock mechanism is released, working hydraulic fluid is supplied repeatedly into the phase-advance side hydraulic chamber and the phase-retard side hydraulic chamber in turn, prior to hydraulically pushing the lock pins back to the housing side.
Therefore, in the Patent document 1, valve timing becomes kept stably at the intermediate lock position, so as to establish a specific state where relative rotation of the housing and the rotor from the intermediate lock position can be suppressed. Hence, it is possible to reduce frictional forces between each individual lock pin and each of the housing and the rotor, caused by the lock pins, each pushed against the housing and the rotor, thereby enabling the two lock pins to be easily pulled out of the respective lock grooves.
However, it is difficult to exactly equalize the hydraulic pressure in the phase-advance side hydraulic chamber with the hydraulic pressure in the phase-retard side hydraulic chamber by supplying working fluid repeatedly to the phase-advance side hydraulic chamber and the phase-retard side hydraulic chamber in turn. Therefore, due to individual differences of component parts, there is a possibility that the frictional forces between these component parts, caused by the lock pins, each pushed against the housing and the rotor, undesirably increase and thus the release of the intermediate lock mechanism fails.
Additionally, in the case of the intermediate lock mechanism having two lock pins, suppose that the hydraulic pressure in the phase-advance side hydraulic chamber is not exactly equalized to the hydraulic pressure in the phase-retard side hydraulic chamber. One of the two lock pins tends to be necessarily pushed against the housing and the rotor. Therefore, there is a very strong possibility that the release of the intermediate lock mechanism may fail.